---

The IPC-2252, "Design Guide for RF/Microwave Circuit Boards"

Nicholas G. Paulter, Jr.National Institute of Standards and Technology

Gaithersburg, MD

AbstractThe IPC-2252, "Design Guide for RFlMicrowave Circuit Boards,,,1(superceding the IPC-D-3l6) was published in July 2002. TheIPC-2252 provides information pertaining to the design, fabrication, and test of printed wiring board used in high-frequency (100MHz to 30 GHz) applications. The design topics include those describing the fabrication procedure and the circuit and boardlayout. Different types of board interconnects and chip mounting strategies arediscussed. This document introduces the IPC-2252and its contents. The differences between the IPC-2252 and IPC-D-3l6 are also discussed and a comparison to other IPCdocuments is provided.

IntroductionThe IPC-2252, "Design Guide for RFlMicrowave Circuit Boards,,,1 is written as a technical guide for high frequency printedwiring boards. This guide contains information on design processes used in the fabrication of these boards, on methods used toattach other electronic components (such as integrated circuits and discrete devices) to the board, on methods to interconnect otherboards, on methods to mount the board into a housing, on the design of the board structure (layering), and on the design of theconductor layout (signal lines and power and ground planes). The IPC-2252 was written to be a guide and not a detailed technicaltreatise on high-frequency printed wiring board technology. Its primary purpose is to provide information on all the requirements,considerations, and steps necessary to successfully fabricate a radio frequency (RF)/microwave printed wiring board (PWB).

BackgroundD-21b Task GroupThe IPC-2252 was written by the D-2l b, the High Frequency Design Task Group. The D-2l b is one ofthree task groups presentlywithin the D-2l, the High Speed/High Frequency Design Subcommittee, which in turn is one of the four subcommittees under theD-20, the High-Speed/High-Frequency Committee. Task groups are formed by the subcommittees, witlrpermission from the IPCTechnical Activities Executive Committee, to address specific tasks that would otherwise consume too much time for the entiresubcommittee membership to address.

The D-2l b membership consists primarily of technical people from the automotive and aerospace industries, instrumentationmanufacturers, material and laminate suppliers, and printed wiring board manufacturers. Consequently, the technical backgroundof the D-2l b membership is quite diverse. This diversity ensures that the IPC-2252 addresses most of the PWB industry concernsfor high-frequency PWBs and that it is written to be understandable by people from many industries.

The latest task of the D-2lb Task Group was to revise the guide, IPC D-3l6, "Design Guide for Microwave Circuit BoardsUtilizing Soft Substrates.,,2 The D-3l6 was published in 1995 and, due to technical advancement in the printed wiring boardindustry, was in need of updating. Furthermore, typographical errors existed in some equations that had caused a bit of confusionwith users of the D-3l6 and some information in the document was not presented clearly.

The D-2l b was assembled to write the D-3l6 and, a couple of years after publishing the D-3l6, the D-21 b decided it wasnecessary to revise the document. Prior to 1998, the activities and business of the D-21b were conducted primarily at two annualmeetings, the IPC Annual Meeting and the IPC Expo. In 1998, the D-21b agreed to use e-mail to exchange files for review, toexamine suggested changes and comments, and to conduct other discussions. The use of e-mail greatly facilitated discussions andexpedited the writing of the IPC-2252, which is the successor to the D-316. (The IPC implemented document renumbering in2000.)

S08-1-1

Intended Audience

As mentioned earlier, the IPC-2252 was written to be a guide and not a detailed technical treatise on high-frequency printed wiringboard technology. Although the IPC-2252 may be used by the expert in their respective field within the umbrella of printed wiringboard technology, it was primarily written to provide general guidance to the novice, information to managers and administrators,and to be used by the experts needing information outside of their field of expertise (such as a laminate manufacturer wanting tounderstand high-frequency signal line design issues).

MICROWAVE CIRCUIT DESIGN FLOWCHART

DESIGN INPUTS

Design OptionsL.iterature Search

List Option Specs.Select Best Option

ADMINISTRATIVE INPUTS

Prel iminary SimulationDetailedMechanical Design

Pc6t-La~'Out SimulationLayout

liG ConsiderationsFabrication/Assembl y

Packaging

Design Review'Producibi lityReliability

Cost Estimate

No

OptimizeDesign

r.t: nS2.3 t

Figure 1 - Microwave Circuit Design Flowchart (Taken from IPC-2252)The IPC-2252ContentsThe IPC-2252 contains 9 sections. The first section provides a list of terms and their definitions. Section 2 contains referenced

S08-1-2

documents,which are mostlyother IPCdocumentsand standardsfor materialsand design.Sections3 and 4 describedesignconsiderations and documentation requirements. Design considerations included are choice of transmission line structure, material,electrical and mechanical designs, and a description of the overall design flow process. Section 5 contains a discussion onmaterials used in high-frequency printed wiring boards for both dielectrics and conductors. Section 6 contains accurate frequency-domain-based mathematical formula describing the electrical characteristics of stripline and microstrip transmission lines, whichare commonly-used transmission line structures in high-frequency circuits. In Section 7, board requirements are discussed and, inSection 8, device attachment and packaging. Section 9 provides a brief description of quality assurance and references IPCdocuments that have more extensive information on this subject.

Section 1, "General"Section 1 of the IPC-2252 contains the purpose and scope of the IPC-2252 and defined terms. As stated in the IPC-2252, thepurpose of the document is to "aid in the design of manufacturable microwave circuit boards." The key word in the scope is"manufacturable," which implies not only realizable, in the sense of being manufactured with available equipment and procedures,but also within cost margins appropriate for typical and common microwave applications. Therefore, the IPC-2252 may notinclude some considerations that are unique to space and military applications.

There are 46 definitions in the IPC-2252. These definitions areused to describe the terms used in the document that may be unique

to and/or used uniquely by the IPC-2252 and/or commonly-used terms for which no acceptable definition was found. Termsdefined include terms for electrical characteristics such as characteristic impedance, effective permittivity, dissipation factor,return loss, scattering parameters; terms for design parameters and features such as coefficient of thermal expansion, copperweight/thickness, ground plane, peel strength, plated-through holes; etc.

Section 3, "Design Considerations"This section is short and basically describes the design process and considerations. The discussion is based on a circuit designflowchart (see Figure 1). As can be seen from the flow chart, the first task is to determine how the board will be used (DesignInputs) and the resources available to manufacture the board (Administrative Inputs). Once these inputs have been determined andare not in conflict, which in essence defines a manufacturable board, the process of producing a manufacturable board can bestarted. The first steps are to provide an initial design and estimate the electrical performance of this board. If these steps aresuccessful, then the design details and specifications are produced and a path for board fabrication, assembly, and packagingdefined. A prototype board is then made and tested and, if theprototype board functions successfully, production boards are made.

Section 4, "Documentation Requirements"Documentation in the fabrication of a printed wiring board (PWB) is necessary for several reasons. First, it allows themanufacturer to track the manufacturing process and effects of this process on board performance. Documentation is also anecessary requirement for most quality management systems. The design feature listing is a document that contains all themanufactured features and processes, such as holes, planes, plating, etching, etc. The master drawing contains all the informationto layout the board. Therefore, each layer of the board must be detailed, showing all conductors, holes, and other features withdimensions (on-plane dimensions as well as thicknesses) and tolerances.

Section 5, "Materials"The materials section is the longest section in the IPC-2252. It contains information on material properties, environmental effects,bonding, plating, etc. The material property that is the most important for RF and microwave PWBs is the relative permittivity, Er,of the substrate. Eris a complex value (E'r.the real part, and E"r,the imaginary part) and usually expressed in the PWB industry interms of the dielectric constant (E'r) and the loss tangent or dissipation factor (E"rlE'r)'Typical values of E'r for RF/microwavePWBs range from 2.1 to 11 (see ref. 1, Table 5-1). The selected value must meet the requirements for the circuit functions andspace limitations. For examples, higher values of E'rincrease capacitance. In transmission line design, with all other parametersremaining the same, an increase in E'rwill decrease the characteristic impedance (Zo) of the transmission line. An increase in E'rwill also cause an increase in the resonant frequency of resonators. The loss tangent is important in both RF/microwave PWBs anddigital PWBs. For the latter, the concern is primarily signal integrity. In RF/microwave PWBs, an increase in loss tangentincreases the power dissipated into the board. Increased power dissipation can cause thermal management problems, especially fordensely packaged boards.

The mechanical properties and thickness of the board affect a variety of board performance parameters. For example, thinsubstrates are not as dimensionally stable as thick substrates. Consequently, it will be more costly to fabricate on thin substratesbecause of the difficulty in maintaining design tolerances. Furthermore, mechanical strength is poorer for thin boards than for thickboards; the thin board will have a greater tendency to warp and bow. Soft substrates have more of a tendency to show these effectsthan rigid substrates. However, ifthe need is to make a conformal board, then the thin board is required. As mentioned previously,the loss tangent causes power dissipation. This dissipation is into the board. Thicker boards will be able to accommodate greater

S08-1-3

power dissipation than thinner boards. Also, high peak powers can cause corona discharge. Circuits made on thicker substrates

will be less likely to fail because of corona discharge than circuits made on thin substrates.

The effect of the processing and operating environment on the board material is also a very important criterion in the selection of aboard material. These effects include not only temperature and humidity, but chemical resistance too. Boards are subject to a largetemperature range and different chemical environments during fabrication. For complex board designs, which typically requiremultilayer boards, the stability of the board material to these environments becomes increasingly important. The board materialmust be able to accommodate these environments without delaminating, warping, etc.

There are other substrate-related properties that are important to the successful fabrication of the RF/microwave PWB and theseinclude, for example, e'r as function of temperature; coefficient of thermal expansion; tensile, compressive, and flexural strengths;and specific gravity.

Bonding films are another component ofthe dielectrics used in RF/microwave PWBs and their characteristics are important to theoperation and function of the circuit. Bonding films are used to glue together layers of dielectric (metal clad or not) to form amultilayer PWB. These bonding films may be thermoplastic or thermosetting in nature. e'r of typical thermoplastic bonding filmsis in the range of2.1 to 2.4 (see ref. 1,Table 5-2) and that of thermosetting bonding films is 3.5 to 4 (see ref. 1,Table 5-3). Thesevalues of e'r may not necessarily match that of the substrate materials, which may make the circuit harder to model and design.

The selection of metal conductors to be used in the RF/microwave PWB is also important. The IPC-2252, in Section 5.3, discussesmany different possible conductors that can be used in the manufacture ofa PWB. The conductors can be plated or clad. For cladconductors, both thin (less than 100 Jlm thick) conductors and thick conductors (greater than 100 Jlffithick) are used. Cladconductors are typically roughened to enhance adhesion to the surface of the dielectric. The effect of this roughening can beobserved more readily in thin claddings than in thick claddings as an increase in conductor loss, which is frequency dependent.

Plated conductors are usually deposited using an electrodeposition process. However, some plated conductors can be depositedusing an electroless method. An electrodeposited metal will require that the dielectric first have a conductive coating, which maybe obtained from an electro less metal deposition. Copper, nickel, gold, silver, and tin are some ofthe electrodeposited metals usedfor conductors in a PWB. Copper and tin can also be deposited using an electro less process. Gold is used primarily because of itscorrosion resistance and high electrical conductivity (4.1x 107S/m).Silver is used because it has the highest electrical conductivity(6.1x 107S/m)of commonly available conductors, but it tarnishes (oxidizes) easily and may migrate into the substrate material.Because of this, nickel is typically used as a diffusion barrier between the silver and the substrate and rhodium is on top of thesilver to prevent oxidation. Copper is the ubiquitous metal used in PWB conductors. Copper has a high electrical conductivity(5.7xl07 S/m), is readily available, and can be plated with both electrolytic and electroless processes. Tin, because its electricalconductivity (9x106 S/m) is relatively low, is primarily used to protect a copper layer.

Conformal coatings are also discussed in the IPC-2252 but are not recommended. Conformal coatings are protective layers that aredeposited on top of conductors. Because these coatings typically have an e'r value different from that of the substrate material, theelectrical performance of the circuit will change. Additionally, the dissipation factor of these coatings is typically greater than thatof the substrate thereby increasing dielectric loss.

Section 6, "Electrical Characteristics"Section 6 contains accurate frequency-domain-based mathematical formulas describing the electrical characteristics of striplineand microstrip transmission lines. These structures are commonly used in RF and microwave circuits. The IPC-2252 defines amicrostrip structure as "A transmission line structure that consists of a signal conductor that runs parallel to and is separated from amuch wider ground plane" and a stripline structure as a "A transmission line structure that consists of a signal line that runsparallel to and is sandwiched between and separated from two wider ground planes." For stripline, both narrow and wide signallines are considered. The characteristics discussed for both types of structures are conductor and dielectric loss, characteristicimpedance, and propagation function. The formulas presented are synthesis type and include the effects of signal line widthrelative to its thickness, finite conductivity of the conductors, and conductor surface roughness. The limits of applicability, interms oftransmission line geometry and material properties, are also given. However, as with most formulas for transmission lineproperties, the ones provided in the IPC-2252 should be used as a tool for understanding the effects of different design parameterson the properties of the transmission line. These formulas may even be useful for the initial design phase (see microwave circuitdesign flowchart shown in Figure 1). For the final design stages of the board, it is recommended practice that electromagnetic-fieldsimulation software be used to improve success of achieving specified or required circuit operation.

Section 7, "Detailed Board Requirements""Board requirements" is a broad category that includes many topics, such as, machined features, imaging, Polytetrafluoroethylene

S08-1-4

(PTFE)considerations,metallization details not discussed in Section 6 ofthe IPC-2252, etching, and bonding. Machined featuresinclude plated-through-holes (PTHs), unplated holes, pockets, slots, and cavities. The aspect ratio, Ra, (depth to diameter) forPTHs defines the difficulty in fabricating a PTH. For example, PTHs with Ra~ 3 are easy to fabricate and those with Ra~ 10arealmost impossible to make. High RaPTHs are more difficult to drill and to maintain plating uniformity along its depth. Hole sizeafter plating may also be difficult to predict because of variations in local plating current densities. Pockets and slots are producedwith tools having a radius. Accordingly, the design for these types offeatures must accommodate the tool used to make them. Thesubsection on machined features also includes a discussion on tooling hole requirements (size, placement, number), the use ofpress-fit pins for making ground connections for metal-backed substrates, and dimensioning and tolerancing.

Under tolerancing, bilateral and true-position tolerancing are compared (see Figure 2). Bilateral tolerancing is where both the xand y tolerances (the coordinate system used to describe features on a panel is a two-dimensional cartestian system, using x and yto denote those dimensions) are defined independently of each other. Bilateral tolerancing, therefore, generates a square tolerancezone. True-positioning tolerancing, on the other hand, provides an elliptical tolerance zone, where the x and y tolerances are themajor and minor axes of an elliptical tolerance zone. It is recommended that true-position tolerancing be used because it is lesslikely to accept poor parts or reject acceptable parts than is bilateral tolerancing.

m 0.010 I0.0 10" circular b:)lerancezone

:t O.OOS'" bilateral(square)tolerancezone

Non-functionalfeaturesconsideredacceptablewhen employingbilateraltoleranclng

m 0.010 I

:t 0.0035" inscribed bilateral tolerance zone

Acceptable features rejected whenemployingInscribedbilateraltoleranclng

IPC-2252-7-1

Figure 2 - Comparison of Bilateral and True-Positioning Tolerancing Methods (Taken from IPC-2252)Guide for generating the artwork and transferring the artwork to the board is discussed in Section 7.2 of the IPC-2252. Negativeartwork is usually used to transfer print and etch patterns and positive artwork for pattern plating. Photo-activated organic films(photoresist) are used to transfer the image from the artwork to the board. The photoresist can be either a dry film or liquid.Thinner resists allow for smaller feature sizes than does thicker resists. However, thinner resists are more susceptible to pinholeformation trom handling and airborne particulates. The designer should not compensate his/her design geometries for the effect ofthe fabrication process. This is because the fabricator knows how to compensate for their process to achieve the circuit given bythe design information and each fabricator may have different compensation factors.

The board manufacturer needs to be aware that, in the etching process, the copper etch rate will vary across the board, dependingon the pattern geometry and conductor thickness. The etch rate of lines will also depend on the proximity of lines. For example,isolated narrow lines separated by a large gap will etch more quickly than a cluster of closely-grouped lines. Undercutting of theetched conductors will also occur. The effect of undercut is greater for thick conductors than for thin conductors. The deposition

S08-1-5

rateand uniformity of electrodeposited copper will also depend on pattern geometry. That is, plating of copper lines in an areawith a lot of conductor coverage should not be expected to plate at the same rate as lines in an area nearly void of conductors.

Section 8, "Device Attachments and Packaging"Section 8 discusses three levels of attachment, namely, attaching the board to the housing, attaching the connector to the board,and attaching the device to the board. Methods for attaching the board to the housing include mechanical attachment (usingvarious types of fasteners), epoxies (conductive and nonconductive), thermoplastic films, soldering, and direct bonding.Mechanical attachment of soft substrates, such as PTFE, should be done with caution as this may deform the board. One way tominimize deformation of PTFE boards is to use metal plates to distribute the force over large areas.

Conductive epoxies are thermosetting resins that are filled with a metal powder, typically silver. Metal filled epoxies tend to bebrittle and do not accommodate differential temperature expansion well. Consequently, these epoxies should be avoided inapplications where there will be a large range of temperatures. Since there are many types of epoxies available, the one best suitedfor the application should be selected. However, bond failure over time may still occur and this may effect the circuit function.Using nonconductive epoxies to secure a board with a ground plane can cause grounding and interconnect problems.

Soldering is the most common method of attaching the completed board to the housing. In this process, both the board and housingare coated with solder and attachment is achieved when the housing is heated above the solder melting temperature. For largeboards, this process may become complicated and require special fixtures to hold the parts in position. Another temperature-basedmethod is called direct bonding. In direct bonding, the board and housing are placed in contact and the temperature is raised abovethe melting temperature of the board material. Pressure is usually applied to the board during the bonding process.

Several different methods for device attachment are mentioned in the IPC-2252. These methods include welded bonds (resistancewelding, parallel gap welding, percussive arc welding, laser welding, and soldering) and diffusion bonding (ultrasonic bonding,thermal compression bonding, and thermosonic bonding). The IPC-2252 provides a brief description ofthese different methods.

Changes relative to the IPC-D-316The IPC-2252 contains the same basic information as the document it superseded, the IPC-D-316. Each major section wasrearranged to provide better continuity within that section and improve readability throughout the document. Redundantinformation was deleted and similar concepts, that were presented differently in two or more locations in the IPC D-316, werecombined. The definitions in the IPC-2252 were rewritten if they where unclear and deleted if they appeared in and wereadequately defined by the IPC-T-50F, "Terms and Definitions for Interconnecting and Packaging Electronic Circuits.") Changeswere made to some definitions to make them consistent with those from other technical organizations, such as the Institute ofElectrical and Electronic Engineers (IEEE). Sections 3 and 4 did not change significantly from the IPC-D-316 other than forimproved readability and update of references. The basic content of Sections 5 through 8 did not change significantly either.However, discussions and tables were updated based on current technology and manufacturing capability, and new informationadded. The errors in the formulas of Section 6 were corrected and the formulas rewritten to more closely parallel those from thereferences. Section 9 was changed significantly. The IPC-D-316 contained much of the same information that is in one of thefollowing specifications: the IPC-6018A, "Microwave End Product Board Inspection and Test,"\ the ANSI/IPC-4103,"Specification for Base Materials for High Speed / High Frequency Applications,"s and the ANSI/IPC-2221, "Generic StandardonPrinted Board Design.,,6.This redundant information was deleted in the IPC-2252 and replaced with references to the appropriateIPC specification.

Comparison to Other IPC documentsThere are four other IPC documents, two guides and two specifications, that have some degree of overlap with or complement theIPC-2252. The guides are all written with the same intention as that for the IPC-2252, that is, to be informative. The guide IPC D-317A, "Design Guideline for Electronic Packaging Utilizing High-Speed Techniques,,,7 (presently being rewritten by the D-21aTask Group and will become the IPC-2251) provides a discourse on topics pertinent specifically to high-speed circuit design.Because of the similarity in some requirements, the IPC-D-317A and the IPC-2252 have related discussion regarding transmissionline electrical characteristics. However, the synthesis formula are different between the two documents because theIPC-D-317A isconcerned with broadband signals (pulses) whereas the IPC-2252 is focused on narrowband signals. Similarly, the majority ofdiscussion is directed toward narrowband signals in the IPC-2252 and pulse signals in the D-317A. Consequently, thesedocuments complement each other well by providing guides to high-frequency and high-speed PWBs. The guide, IPC-2141,"Controlled Impedance Circuit Boards and High Speed Logic Design,"S (presently being rewritten by the D-21c Task Group)describes topics specific for controlled impedance transmission lines. Controlled impedance typically implies impedancetolerances of:!: 10% or less. There is not significant overlap in the contents of the IPC-2252 and the IPC-2141 other than for boardand transmission line design. The IPC-60 18A is a specification and, therefore, prescriptive or normative, whereas the IPC-2252 isa guide, an informative document. These two documents contain unique information and together form a more completeinstruction on high-frequency PWBs, from design concept to test. The other document that has some relation to the IPC-2252 is

S08-1-6

the ANSIIIPC-4103. The ANSIIIPC-4103 is a nonnative document that provides tables of requirements for different laminates andinterlayer adhesives used in high-speed/high-frequency PWBs. Although the IPC-2252 provides certain suggested requirementsfor laminates used in the fabrication of high-frequency PWBs, it does not provide the detailed requirements contained in the IPC-4013.

Future of the IPC-2252Because PWB technology changes rapidly, revisions of published documents are often considered immediately after their release.People interested in participating in a subsequent revision of the IPC-2252 should contact John Perry of the IPC(iohnDeITVca2iDc.ore)and indicate this interest and, to ensure their participation, ask to have their names added to the D-21subcommittee and D-21b task group rosters.

AcknowledgementNicholas G. Paulter, Jr. - Electricity Division, Electronics and Electrical Engineering Laboratory, Technology Administration,Department of Commerce. Official contribution of the National Institute of Standards and Technology, not subject tocopyright in the U.S.A.

Conclusion .

The IPC-2252 supercedes the IPC-D-136 and was published in July 2002. The IPC-2252 was introduced herein by describing itscontents. The differences between the IPC-2252 and IPC-D-316 were also discussed and a comparison to other IPC documentsprovided.

References

1. IPC-2252, Design Guide for RF / Microwave Circuit Boards, IPC, Northbrook, IL, July 2002.2. IPC-D-316, Design Guide for Microwave Circuit Boards Utilizing Soft Substrates," IPC, Northbrook, IL, June 1995.3. IPC-T-50F, Terms and Definitions for Interconnecting and Packaging Electronic Circuits, IPC, Northbrook, IL, June 1996.4. IPC-60 18A, "Microwave End Product Board Inspection and Test," IPC, Northbrook, IL, January 2002.5. ANSI/IPC-4103, "Specification for Base Materials for High Speed / High Frequency Applications," IPC, Northbrook, IL,

January 2002.6. ANSI/IPC-2221, Generic Standard on Printed Board Design, IPC, Northbrook, IL, February 1998.7. IPC-D-317 A, Design Guidelines for Electronic Packaging Utilizing High-Speed Techniques," IPC;-Northbrook, IL, January

1995.

8. IPC-2141, "Controlled Impedance Circuit Boards and High Speed Logic Design," IPC, Northbrook, IL, April 1996.

S08-1-7

MINNEAPO~ISCONVENTIONCENTER'- M~NEAPOLlS,MN

IPC s AnnuallVleetingSEPTEMBER 28-oCTOBER 2, 2003

(!Pi;;ASSOCIATION CONNECTING

rELECTRONICS INDUSTRIES ~